UV APPARATUS FOR FLUID TREATMENT

Abstract: An ultra-violet water sterilisation system comprises an elongate treatment chamber (10) having an axial ultra-violet lamp (14) arranged to output light having a wavelength of 245-265 nm. The skin temperature of the lamp (14) is maintained at a constant level by energising a fan and/or a heater, which are controlled by temperature sensors (21, 22). A microprocessor (28) calculates the dose of radiation delivered to the water from the product of the UV intensity and the exposure time. The UV intensity is measured using a photodetector head (23) mounted to the side of the chamber (10). The photodetector head (23) comprises a reverse biassed vacuum photodiode which acts as a sensor for UV light in the wavelength 245-265 nm.

Description: UV APPARATUS FOR FLUID TREATMENT This invention relates to fluid treatment and more particularly but not solely to a system for sterilising water or sewage. It is well known that high-intensity ultra-violet (UV) light has germicidal properties which can be used to sterilise water. UV sterilisers are not widely used by water treatment companies, because they do not effectively treat all of the water. Often micro-organisms can pass through the steriliser without being affected by the UV light. This can happen when the micro-organisms are not exposed to high-intensity UV light for a sufficiently long period of time. It is not practical to continuously monitor the treated water for live micro¬ organisms, and thus there is no way of knowing whether the system is working effectively. We have now devised a fluid treatment system which effectively treats the fluid being treated. In accordance with this invention, as seen from a first aspect, there is provided a fluid treatment system comprising a treatment chamber having a source of radiation directed at the fluid being treated, means for determining the intensity of the radiation emitted by the source, means for determining the length of time for which the fluid being treated is exposed to the radiation in said chamber and means for determining the dose rate of the radiation applied to the fluid being treated, using the determined values of the intensity of the radiation and the exposure time. Thus, the determined dose rate can be compared with a minimum acceptable dose rate in order to determine whether the system is working effectively. Preferably the determined value of the dose rate is used to control a fluid flow valve, so that the fluid being treated receives the correct dose of radiation. Preferably the system comprises a microprocessor which calculates the exposure time using a predetermined value of the volume of the treatment chamber and a measurement of the fluid flow rate. Preferably means are provided for monitoring electrical current drawn by the source of radiation in order to determine whether the current is too high or too low. Preferably means are provided for monitoring the run time of the source of radiation, and for giving an indication of the length time for which it has been operating, or for giving an indication of the length of time remaining before its period of operation exceeds its life expectancy. Preferably the source of radiation comprises an ultra- violet lamp arranged to output light which has germicidal properties. The germicidal effect of UV light occurs at a wavelength of 245-265 nM, and therefore it is important to ensure that the light source maintains a high output within this wavelength range, otherwise micro-organisms can pass through the steriliser without being affected by the UV light. Some types of UV lamps, such as low pressure mercury arc lamps are highly sensitive to changes in their skin temperature, and we have found that it is important to ensure that the lamp skin temperature remains substantially constant so that the output of the light source, in the wavelength range mentioned above, remains high. Thus means are preferably provided to monitor the surface temperature of the lamp and to actuate heating or cooling means, in order to maintain the lamp substantially at its optimum working temperature. Preferably the system comprises an alarm which operates when the system is found to be working outside its normal operating parameters. Preferably the system is arranged to connect further treatment chambers into the fluid flow path when the minimum acceptable dose rate is reached, or when the fluid flow rate falls below a predetermined lower level. Preferably the radiation monitoring means monitors the output of the UV light at 245-265 nM, in order to determine whether the steriliser is working effectively. However, most types of photodetectors used for monitoring light output are not sufficiently sensitive at the lower end of the UV spectrum without complicated and expensive optical filters to select the germicidal wavelength. Solid state silicon photodiodes are a well known type of photodetector having a broad spectral response covering the germicidal wavelengths, however these photodiodes tend to loose their sensitivity gradually with age. This so-called exposure ageing is particularly worse under continual exposure to UV radiation, and thus complicated compensating electronic circuitry is required. Other expensive and complicated laboratory type photodetector apparatus are available which can be tuned to accurately measure the intensity of particular wavelengths of light. However, these apparatus are not suitable for industrial applications owing to their high cost. Thus, in accordance with this invention, as seen from a second aspect, there is provided a photodetector apparatus comprising a photodiode and means for applying a reverse bias to the photodiode and for monitoring the magnitude of the reverse bias current flowing through the photodiode, so as determine the value of light incident on the photodiode. Although photodiodes have a broad spectral response band in their conventional forward bias mode of operation, we have found that in the reverse bias mode they have a narrow spectral response band. A spectral response to suit most needs can be obtained by selectively choosing the type of photodiode. Preferably the photodiode comprises a vacuum photodiode preferably having a CsTe photosensitive cathode. CsTe vacuum photodiodes used in the reverse bias mode have a spectral response range of 185 nM to 327 nM, and thus are particularly sensitive to UV at germicidal wavelengths i.e. 245-265 nM. Vacuum photodiodes do not suffer from exposure ageing when operated in the reverse bias mode. They also have the advantage of a linear output, which remains stable across a wide range of temperatures. Preferably the photodiode is connected to a circuit having a high input impedance, which does not load the device. Preferably the photodiode is mounted in a housing comprising a transparent wall having a relatively adherence- resistant material on its outer surface. The adherence- resistant material prevents slime and other waste matter from building up on the transparent wall when the apparatus is used to measure the amount of light transmitted through a fluid. Preferably the material comprises a fluorocarbon polymer such as PTFE. As stated above, low pressure mercury arc lamps are highly sensitive to changes in their skin temperature, and consequently their light output in the range 245-265 nM is much reduced if they depart from an optimum temperature. Therefore the sterilising system will not work effectively if the fluid being sterilised is at a temperature above or below this optimum lamp temperature. GB 2 175 779 discloses a water purifier comprising an elongate UV lamp disposed within a quartz glass sleeve. A heating element is wound around the lamp in the air space between the lamp and the sleeve. The heating element is controlled by a thermistor which energises the element when the temperature of the lamp falls below its optimum working temperature. The arrangement of the heating element around the lamp is difficult to assemble, furthermore it is difficult to withdraw the lamp from the sleeve without damaging the heating element. Also, the heating element shades some of the water from the UV light, therefore reducing the effectiveness of the treatment. Thus, in accordance with this invention, as seen from a third aspect, there is provided a fluid treatment apparatus comprising a flow passage for the fluid to be treated, a light source for directing light at the fluid in said flow passage through a transparent wall of the flow passage, means for directing a flow of air or other fluid over the light source, a temperature sensing means associated with the light source, and control means responsive to the temperature sensing means to control the flow of air or other fluid over the light source, so as to stabilise the temperature of the light source. Thus, by controlling the flow of the air or other fluid in thermal contact with the light source, the temperature of the light source is stabilised. This method of controlling the flow is efficient and does not cause any shading of the fluid being treated. Preferably the means for directing a flow of air or other fluid over the light source includes a heating means for heating the air or other fluid. Preferably the control means controls the heating means in response to the temperature sensing means. For example, a fan may direct a flow of air or other fluid over the light source without energisation of the heating means when the light source requires cooling, the heating means being energised to warm the flow of air or other fluid when the light source requires heating. In particular, the air or other fluid may flow over the light source without energisation of the heating means when the temperature sensing means indicates a temperature above an upper preset level, the heating means being energised when the temperature sensing means indicates a temperature below a lower preset level: when the sensed temperature is between these two preset levels, preferably the flow of air or other fluid is halted (i.e. both heating means and fan are de-energised). Preferably the light source is disposed within a duct through which the flow of air or other fluid passes. Preferably the light source comprises a mercury arc UV lamp. Preferably the flow of air is circulated over the light source. Embodiments of this invention will now be described by way of examples only, and with reference to the accompanying drawings, in which: FIGURE 1 is a block diagram of a water treatment system; FIGURE 2 is a sectional view through an ultra-violet water treatment chamber of the system of Figure 1; FIGURE 3 is a sectional view along the line III-III of Figure 1, showing a photodetector head of the system in greater detail; FIGURE 4 is a graph showing the spectral response of the photodiode of the photodetector head of Figure 3 ; FIGURE 5 is a schematic diagram of a signal processing circuit of the photodetector head of Figure 3; FIGURE 6 is a graph of output voltage against radiation intensity of the circuit of Figure 5; and FIGURE 7 is a sectional view through an alternative ultra-violet water treatment chamber for use in the system of Figure 1. Referring to Figure 1 of the drawings, there is shown a water treatment system which comprises an ultra-violet water treatment chamber 10. Referring also to Figure 2 of the drawings, the ultra-violet water treatment chamber 10 comprises an elongate tubular duct 11 having inlet and outlet ports 12,13 mounted at its opposite ends. An elongate ultra-violet lamp 14 is mounted inside a quartz glass sleeve 15 which extends along the axis of the duct 11. The sleeve 15 is sealed at its opposite ends to the end walls of the duct 11. Box-shaped compartments 16,17 are mounted to opposite ends of the duct 11, and are arranged to receive the opposite open ends of the quartz glass sleeve 15. A tube 18 co-extends with the duct 11 between the two compartments 16,17. The tube 18 may be provided with cooling fins (not shown) . A fan 19 is mounted at one end of the tube 18 and a tubular cartridge heater 20 projects axially into the tube 18 from its opposite end. A temperature sensing element 21 is mounted inside the quartz glass sleeve 15 adjacent its one end, so that it is shaded from direct UV light which may deteriorate the element. A second temperature sensing element 22 is mounted adjacent the electrical contact terminal at one end of the lamp 14. A photodetector head 23 is mounted on the external wall of the duct 11 and is directed towards the lamp 14. An electrically operated valve 24 is connected to the inlet port 12, and a motorised fluid flow control valve 25 is connected to the outlet port 13. A fluid flow measuring device 26 is connected in the outlet duct upstream of the flow valve 25. A current transformer 27 is arranged around an electrical supply wire to the lamp 14. A microprocessor 28 has inputs connected to the fluid flow measuring device 26, photodetector head 23, temperature sensing elements 21,22 and current transformer 27. Outputs from the microprocessor 28 are connected to the valves 24,25 at the inlet and outlet ports 12,13 to the chamber 10. A keypad 29 is connected to the microprocessor 28 for inputting data. Dial- type switches 30,31 are connected to the microprocessor 28 for setting the minimum acceptable dose of radiation and flow rate limits respectively. Alternatively these limits may be entered using the keypad 29. A chart recorder 32 and a digital display 33 are connected to the microprocessor 28. An audible alarm 34 is also connected to an output of the microprocessor 28. In use, water flows along the duct 11 between the inlet and outlet ports 12,13 of the chamber 10. The UV lamp 14 is energised and emits UV light so that the water flowing through the duct 11 is exposed to UV light. The volume of the chamber 10 is pre-programmed into the microprocessor 28 e.g. using the keypad 29. The flow rate of the water is measured by the device 26 and fed into the microprocessor 28, so that the length of time which the water being treated is exposed to the UV light can be calculated from the following formula: Exposure Time = Volume of the chamber = Seconds Flow Rate The photodetector head 23 monitors the intensity of the UV light at the wall of the duct. The output from the photodetector head 23 is connected to the microprocessor 28 so that the dose rate of the system can be calculated from the following formula: Dose Rate = UV Intensity x Exposure Time = mW Sec/CM 2 The microprocessor 28 compares the instantaneous value of the dose rate with the preset minimum acceptable dose rate. If the dose rate is too low, then the microprocessor outputs a signal to the motorised fluid flow control valve 25 in order to reduce the water flow rate through the system, thereby increasing the dose rate. Thus, a lethal dose of radiation is always delivered. When the flow rate of the water reaches the minimum preset value, the valve 24 operates to either shut down the system or to connect further treatment chambers in parallel with the supply. The digital display 33 gives a reading of the prevailing dose rate of the system, together with an indication of the time remaining until the operating time of the lamp 14 exceeds its recommended life expectancy. The operating time of the lamp may be derived from the output of the photodetector head 23. The microprocessor also monitors the current drawn by the lamp 14 using measurements taken from the current transformer 27. The temperature sensing elements 21,22 are connected to the microprocessor 29, which compares the detected temperatures with preset levels at which the fan and heater or fan only are energised. The preset levels may be adjusted according to the optimum working temperature of the lamp. In the case of mercury arc lamps having an optimum working temperature of 43°C, the preset levels may be 41°C and 45°C respectively. In use, if the water being treated is cold, the air surrounding the UV lamp 14 13 inside the quartz glass sleeve 14 is chilled by the water. Once the temperature inside the sleeve falls below the lower preset level e.g. 41°C, the fan 19 and heater 20 are energised to circulate warm air along the quartz glass sleeve and over the surface of the lamp 14. The air is returned through the tube 18 so that the same warm air is continuously circulated. If the temperature of the water is higher than the upper preset level e.g. 45°C, then the fan only is actuated to circulate cold air over the surface of the lamp 14. The fan or fan and heater continue to operate until the temperature inside the quartz glass sleeve 15 is at a temperature between the upper and lower preset levels. Thus the UV lamp 14 operates continuously within +/- 2°C of its optimum working temperature. The audible alarm 34 outputs an alarm signal when the lamp current is too high or too low, or when the run time of the lamp has exceeded its life expectancy or when the temperature of the lamp or of its end connector is too high or too low. The limits for these operating parameters are pre-programmed into the microprocessor 28 using the keypad 29. The system is able to effectively treat all micro organisms in the water by ensuring that the water always receives greater than the minimum dose of UV light at germicidal wavelengths that is required to kill them. The "photodetector head 23 is mounted at the furthest point away from the lamp 14 so that the worst-case dose rate is used in the calculations. Micro-organisms nearer the lamp 14 will receive greater than the minimum specified dose. It will be appreciated that the system is fully automatic, thereby ensuring that a correct (lethal) dose of radiation is always delivered to the microorganisms. Water treatment companies can therefore have confidence that the system is working effectively. Referring to Figure 3 of the drawings, there is shown in greater detail the photodetector head 23 fitted to the outside of the duct 11 of the ultra-violet water sterilisation chamber 10, such that it receives light emitted from the lamp 14. The head 23 comprises a housing having a tubular portion 35 attached at one end to the duct 11. The tubular portion 35 is closed at one end by a window which comprises an optical attenuator 36 having a layer 37 of PTFE on its outer surface. The optical attenuator 36 has a step-down ratio in excess of 1500:1, and is manufactured by coating a disc of synthetic fused silica with several fine coatings of an alloy of NiCr, e.g. by sputtering or evaporation, until the desired step-down ratio is achieved. The housing further comprises a body portion 38 which screw threads into the opposite end of the tubular portion 35. A vacuum photodiode 39 having a CsTe cathode is mounted axially in the body portion 38 behind the optical attenuator 36. " The outer end of the photodetector head comprises a compartment 40 which contains an electronic circuit 41 connected to the vacuum photodiode 39. A connector 42 is provided on the wall of the compartment for connecting the circuit 41 to the microprocessor 28 and power supply circuits of the treatment system. ■ In use, as water flows along the duct 11 between the inlet and outlet ports 12,13 of the chamber 10, the flow of water creates a scrubbing action which cleans the surface of the PTFE covered optical attenuator 36. Furthermore, the PTFE layer 37 also prevents slime etc. from building up on the attenuator 36. Referring to Figure 4, there is shown a graph of the spectral response of the vacuum photodiode 39 mounted inside the photodetector head 23. The processing circuit 41 provides the vacuum photodiode 39 with a reverse bias, so that only a reverse leakage current flows between its electrodes. This reverse leakage current flows when UV light having a wavelength between 185-327 nM is incident in the photodiode. The UV lamp 14 outputs light at 253.7 nM, which is close to the point of maximum sensitivity of the reverse biassed photodiode. Referring to Figure 5, the photodiode 39 is connected in series with several high value resistors R1,R2,R3 across the supply, so as to form a potential divider circuit. A point P on the potential divider is connected to the non-inverting input of an operational amplifier IC1, which is configured as a buffer amplifier. IC1 comprises a CMOS device having a high input impedance which does not load the photodiode 39, and which provides impedance matching between the photodiode and the output of the circuit 41. The output of IC1 is connected to a potential divider circuit R4 ,R5, which steps down the magnitude of its output voltage. A capacitor Cl connected across resistor R5 smooths the signal at the junction of the two resistors R4,R5, so as to form a DC level which varies in magnitude depending upon the intensity of UV radiation at wavelengths of between 185 - 327 nM. A second operational amplifier IC2 is configured as a non-inverting amplifier, the gain of which is adjusted by VR1 to give a DC voltage of 27 mV per mW/CM 2 radiation intensity. Referring to Figure 5, the output from the circuit 41 is directly proportional to the UV radiation incident on the photodiode 39, and thus complicated processing circuits are not needed owing to the linear output. Referring to Figure 7, there is shown an alternative ultra-violet water sterilisation chamber for use in water treatment system of Figure 1, and like parts are given like reference numerals. A fan 43 is mounted in the compartment 16 to draw in air from outside the apparatus. A heating element 44 is mounted in front of the fan 43. The fan 43 and heating element 44 are controlled by a temperature sensing element as before, so as to direct cool or warm air along the quartz glass sleeve 15, and thereby stabilise the air temperature inside the quartz glass sleeve. The exhaust air is vented through a grille 45 in the compartment 17. In this way the skin temperature of the lamp 14 is controlled to ensure maximum UV output in the 254 nM range.

Claims: CLAIMS 1) A fluid treatment system comprising a treatment chamber having a source of radiation directed at the fluid being treated, means for determining the intensity of the radiation emitted by the source, means for determining the length of time for which the fluid being treated is exposed to the radiation in said chamber and means for determining the dose rate of the radiation applied to the fluid being treated, using the determined values of the intensity of the radiation and the exposure time. 2) A fluid treatment system as claimed in claim 1, in which the determined the determined value of the dose rate is used to control a fluid flow valve. 3) A fluid treatment system as claimed in claims 1 or 2, comprising a microprocessor which calculates the exposure time using a predetermined value of the volume of the treatment chamber and a measurement of the fluid flow rate. 4) A fluid treatment system as claimed in any preceding claim, comprising means for monitoring electrical current drawn by the source of radiation, and for determining whether the monitored current is too high or too low. 5) A fluid treatment system as claimed in any preceding claim, comprising means for monitoring the run time of the source of radiation, and for giving an indication of the length time for which it has been operating, or for giving an indication of the length of time remaining before the period of operation of the source of radiation exceeds its life expectancy. 6) A fluid treatment system as claimed in any preceding claim, in which the source of radiation comprises an ultra¬ violet lamp. 7) A fluid treatment system as claimed in claim 6, in which the ultra-violet lamp radiates light having a wavelength Of 245-265 nM. 8) A fluid treatment system as claimed in claim 7, in which the radiation monitoring means monitors the output of the UV light at 245-265 nM. 9) A fluid treatment system as claimed in any preceding claim, comprising means for monitoring the surface temperature of the lamp and arranged to actuate heating or cooling means, in order to maintain the lamp substantially at its optimum working temperature. 10) ,A fluid treatment system as claimed in any preceding claim, comprising an alarm which operates when the system is found to be working outside its normal operating parameters. 11) A fluid treatment system as claimed in any preceding claim, arranged to connect further treatment chambers into the fluid flow path when the minimum acceptable dose rate is reached, or when the fluid flow rate falls below a predetermined lower level. 12) A photodetector apparatus comprising a photodiode and means for applying a reverse bias to the photodiode and for monitoring the magnitude of the reverse bias current flowing through the photodiode, so as determine the value of light incident on the photodiode. 13) A photodetector apparatus as claimed in claim 12, in which the photodiode comprises a vacuum photodiode. 14) A photodetector apparatus as claimed in claim 13, in which the vacuum photodiode has a CsTe photosensitive cathode. 15) A photodetector apparatus as claimed in any of claims 12 to 14, in which the photodiode is connected to a circuit having a high input impedance. 16) A photodetector apparatus as claimed in any of claims 12 to 15, in which the photodiode is mounted in a housing comprising a transparent wall having a relatively adherence- resistant material on its outer surface. 17) A photodetector apparatus as claimed in claim 16, in which the material comprises a fluorocarbon polymer. 18) A fluid treatment apparatus comprising a flow passage for the fluid to be treated, a light source for directing light at the fluid in said flow passage through a transparent wall of the flow passage, means for directing a flow of air or other fluid over the light source, a temperature sensing means associated with the light source, and control means responsive to the temperature sensing means to control the flow of air or other fluid over the light source, so as to stabilise the temperature of the light source. 19) A fluid treatment apparatus as claimed in claim 18, in which the means for directing a flow of air or other fluid over the light source includes a heating means for heating the air or other fluid. 20) A fluid treatment apparatus as claimed in claim 19, in which the control means controls the heating means in response to the temperature sensing means. 21) A fluid treatment apparatus as claimed in claims 19 or 20, comprising a fan which directs a flow of air or other fluid over the light source without energisation of the heating means when the light source requires cooling, the heating means being energised to warm the flow of air or other fluid when the light source requires heating. 22) A fluid treatment apparatus as claimed in any of claims 18-21, in which the light source is disposed within a duct through which the flow of air or other fluid passes. 23) A fluid treatment apparatus as claimed in any of claims 18 to 22, in which the light source comprises a mercury arc UV lamp . 24) A fluid treatment apparatus as claimed in any of claims 18 to 23, in which the flow of air is circulated.

Inventors: SNOWBALL MALCOLM ROBERT, SNOWBALL, MALCOLM, ROBERT
Applicants: SNOWBALL MALCOLM ROBERT, WATER RECOVERY PLC, SNOWBALL, MALCOLM, ROBERT
Application year: 1994
Main classification: C02F1
IPC codes: B01J19, C02F1, G01J1